# Finding the Optimal Combination of Resistors for a Desired Total Resistance

• a1234
In summary, the conversation discusses finding a combination of resistors with specific values that will add up to a total resistance of 62.5 Ω. The participants share their attempts and strategies for solving the problem, including using the equations for parallel and series resistors and noticing patterns in the values. They also discuss the process of finding the solution and the usefulness of creating a notation for the circuits.
a1234

## Homework Statement

I'm asked to find a combination of resistors (parallel and/or series) that uses resistors of 25 Ω, 100 Ω, 50 Ω, and 50 Ω. They should add up to give a total resistance of 62.5 Ω.

## Homework Equations

Req for parallel = 1/R1 + 1/R2 + ...
Req for series = R1 + R2 + ...

## The Attempt at a Solution

This seems like a relatively simple problem, but I've been unable to find a combination that gives the desired resistance. I noticed, however, that if the 50 Ω, 50 Ω, and 25 Ω resistors are placed in parallel, we get an equivalent resistance of 12.5 Ω, which may help in getting to the total of 62.5 Ω. I'm also required to use all the resistors.

Can you think of a way to make the numbers smaller?
Since 100>62.5, what will you need to do with that one?

I hate this problem. I also failed to find a combination that works. I can find it if the 100 was a 50 or if there was another 100 (so the two 100s could be combined in parallel for 50). Maybe there is one. @haruspex ,without giving away too much, do you see a combination that works?

FactChecker said:
do you see a combination that works?
Yes, it has a solution.
There is a methodical approach, but it is a bit tedious.

FactChecker
haruspex said:
Yes, it has a solution.
There is a methodical approach, but it is a bit tedious.
Thanks.
@a1234 , has your class discussed any procedure to solving these?

I finally found it. After a1234 has solved it, I would like to know about a more methodical way, tedious or not, unless it is just trying all combinations. Even "branch and bound" would be some help.

Last edited:
FactChecker said:
do you see a combination that works?
Yes, there is a solution.

But I don't know what hint I should give without accidently giving away the answer. Will post later when I have the right words for that hint.

Edit: I see Factchecker has also found it.

FactChecker
the best way i found to working out the solution was first assuming that the 100 is in a parallel circuit.then it was a matter of calculating as i went to see if i was getting higher or lower than 62.5, thus determining what had to be arranged to get higher or lower.

honestly tho, this question to me was really a furious game of mix and match with little in the way of logically determining your steps beyond that. there is a question, but it really isn't satisfying when you get there, just the relief that you're done.

FactChecker
Pick one resistor and call it R.

Find the resistance Rs that needs to be connected in series with R which gives Req=62.5 ohms.
Try all the combinations of the remaining three resistors to find which one gives Rs.

If that doesn't work,
find the resistance Rp that needs to be connected in parallel with R which gives Req=62.5 ohms. (Here, Rp can only be 100 ohms).
Try all the combinations of the remaining three resistors to find which one gives Rs.

Using some common sense, you can mentally work things out when you are dealing with the remaining three resistors.

Last edited:
FactChecker
I finally solved it. I think the most helpful thing is to notice that the equivalent resistance in a parallel circuit is less than each of the individual resistors.

Thanks for all the help!

FactChecker
a1234 said:
the most helpful thing is to notice that the equivalent resistance in a parallel circuit is less than each of the individual resistors.
Yes, that is what I used, and conversely that the max resistance that can be made from a set is all in series.

My first step was to factor out to reduce the problem to creating a 5 from 2, 4, 4, 8.
It also helps to develop a notation.
I write _ for series and | for parallel, and [ ... ] to denote an unspecified circuit built from the resistors listed.
The outermost structure is two paths in parallel or two paths in series.

We can immediately rule out 2|[4,4,8] and 4|[2,4,8] as being <5.
8|[2,4,4]≤8|10<5.
[8,2]|[4,4]≤10|8<5.
Etc.
So that rules out outermost parallel.
Working through the "outermost in series" cases is tougher.

I suspect there is some Diophantine approach, but it is too subtle for me.

FactChecker

## 1. What is the difference between a parallel and series circuit?

A parallel circuit is a circuit in which the components are connected side by side, allowing the current to flow through multiple paths. In contrast, a series circuit is a circuit in which the components are connected one after the other, creating a single path for the current to flow through.

## 2. Which type of circuit is more efficient?

In terms of efficiency, a parallel circuit is more advantageous as it allows the current to flow through multiple paths, reducing the overall resistance and increasing the flow of current.

## 3. Can you have a combination of parallel and series circuits in one circuit?

Yes, it is possible to have a combination of parallel and series circuits in one overall circuit. This is known as a complex circuit and is commonly used in electrical systems.

## 4. How does the voltage and current vary in a parallel circuit?

In a parallel circuit, the voltage remains constant across all components, while the current is divided among the branches of the circuit, depending on the resistance of each branch. This means that the total current in a parallel circuit is equal to the sum of the currents in each individual branch.

## 5. What are some real-life examples of parallel and series circuits?

Parallel circuits are commonly found in household appliances, Christmas lights, and car headlights. Series circuits are used in simple flashlights, holiday light strands, and old-style Christmas tree lights.

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